This Research Career Award Application describes a mentored research program for the candidate, Dr. Costi Sifri, in bacterial pathogenesis and immunology. The program will be facilitated by supplemental coursework in microbiology, genetics, immunology, and Caenorhabditis elegans biology at Harvard Medical School, the Harvard School of Public Health, and Cold Spring Harbor. The candidate is trained in Infectious Diseases and proposes a research program to study virulence mechanisms of and host response to Staphylococcus aureus infection. The candidate's sponsor, Dr. Stephen Calderwood, and cosponsor, Dr. Frederick Ausubel, have a longstanding collaborative interest in using simple organisms as model hosts for the study of human bacterial pathogens. The candidate has developed a novel system for studying genetic and molecular mechanisms of S. aureus pathogenesis and host defense mechanisms, using the nematode model organism C. elegans. He has found that C. elegans die by an active infectious process when exposed to live S. aureus. Mutants of several S. aureus pathogenicity-related genes, including the virulence regulator agr, the alternative sigma factor sigB, and the V8 protease gene sspA, are significantly attenuated in their ability to kill adult nematodes, suggesting that the molecular basis of S. aureus pathogenicity may be conserved in evolutionary distant hosts. The Specific Aims are as follows: 1) to use the C. elegans model system to screen a library of S. aureus transposon insertion mutants and identify clones attenuated in both nematodes and a mouse sepsis model, 2) to perform in-depth molecular genetic analysis for one or more selected S. aureus mutants, and 3) to identify and characterize C. elegans mutants with altered susceptibility to S. aureus infection. The studies are designed to contribute to our understanding of S. aureus pathogenesis and explore primitive mechanisms of host defense. In addition, these studies will concurrently provide the candidate with the training and experience necessary to develop an independent research career. [unreadable] [unreadable]